Signal generating system



FiledMay 27, 1937 2 Sheets-Sheet 1 ATTORNEY H. M. LEWIS SIGNAL GENERATING SYSTEM Filed May 2?, 1957 2 sheets-shane FIG. 2.

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ATTORNEY Patented July 1, 1941 SIGNAL GENERATING SYSTEM` Harold M. Lewis, Great Neck, N.V Y., assgnorto Hazeltine Corporation,A a corporation` of` Dela- Ware Application` May 27, 1937, Serial No. `145,024

( 01.` Tis- 7.2)

2 Claims.

This invention relates to cathode-ray tube television transmittingV systems and,` more particularly, to an :improved Amethod of, and means for, generating` signals in such systems.

Inl-accordance with present television practice, a-.cathode-ray; signal-generating tube isV employed atv the `transmitter, the cathode ray being deiiected to scan, in a. series of parallel lines,4 a photosensitive target upon whichan image of the scene-.to--ba transmittedLis projected. For this purpose, the-ray..is-ordinarily deected horizontally at are-latively high frequency and verticallyfat a relatively low frequency, the horizontal scanning frequency determiningthe number of lines;per second andthe .'vertical. scanning frequency determining the number of frames per Second. Duringfeach line vdeliection` or trace, the electrostatic potentials: ofthe successive elements ofthe target, which depend upon the light values at correspondingelementsof the scene, are modiedby the cathode ray and video-frequency voltagesare thereby developed-.which are-representativexof I-these varyinglight values `in the successive elementalareas of ,the scene.- The cathode ray is preferably cutsoi or blocked 1out during the line and V:frame-retrace periodsfso as not to aiect thefscreena Despite the-factthatthe ray is cut offfduring `the retrace periods,` however, undesirable electrical impulses r are developed which, if included as a part of the transmitted signal, seriously` impair `the reproduction` thereof at a receiver; Such impulses are` occasioned by various causes-inherentfin theoperation of conventional cathode-ray.4 tubev signal-generating apparatus, somefof which arehnotfully understood.

A, so-called background-illumination signal is also ,developed at v the transmitter and` comprises a-*unidirectional voltage proportional to theaverage:backgroundiillumination ofV the scene to be transmitted.` When the -signal is passed through alternating current,r ampliers, they unidirectional Voltage is, ofcourse, lost.' Itis essential, therefore, in Vtransmitting systems of the character described1that-meansbe provided whereby the undesired impulses` developed during the retrace scanning periods may be suppressed from the signal andtheseveralsignal components to be transmitted may` be so combinedv that the `resultant modulation `signal .includes a background-illumination component; whichmay be utilized at a receiver -to develop yaxunidirectional control voltagerepresentative Aof the average background illumination `of the transmitted-scene.

Heretofore Various systems have been devised in attempt-st ,to accomplish the essential requirements mentioned above. For various reasons, however such systems `have not proved rentirely satisfactory. Eorexample, in these systems ofthe prior art, itl hasbeen found extremely diicult completely to suppressY `the undesired ,retracepericd'impulses and to obtain a sharp cutoff-oithat partei the` signal representative ofthe blacker thanblack` level,wherebywthe signal will correspend. to blackduringall retrace periods. s

It `is an obiectozf the present inventionthere-4 fore, to provide a Cathode-raytube signal-gen'- eratingisystem of the Vcharacter described, whereby. there maybe developed an,undistortedrsignal substantiallyffree from undesired impulsiesoccurring duringL the Yretrace vscanning periods and which includes ra background-illumination'modulation-signal component.

In accordance with .the present invention, there is provided ina `television ,transmitting system, a cathode-ray tube signal-generatingsystem comprising. means .including a scanning system for effecting traoesgandwretraces of thefrayon the target. of the tube for ldeveloping-video-signal voltages, means for developing -Y pedestal-signal voltages during retrace-scanning periods,` and means for developing. a separateaperiodic bias voltage. having an amplitude varying solely in accordance with background illumination.'l A vacuum-tube `amplifier is provided which `includes n a plurality of Vvcontrol,electrodes andan 4output circuit, `together with` means forapplying the videoesignal voltagesto one ofjthe electrodes, means-for applying the pedestal signals-'to one of the electrodes..to4 reduce, the transconductance 5 of the tube substantially, to `z ero duringv periods corresponding. to the retrace-scanning periods, and means` forl applying", the developed background-illumination,,voltage as a bias `to Athe rst one ofthecontrolelectrodes to adjust Vthe zero axis of the wave applied to the amplifier. By such an arrangement, there is developedinrthe output circuit of the vacuum-tube'amplifier a composite, signal including the VideOsignaIfand the 1 background-illumination voltages.

For. a .betterunderstanding of the invention, together-with other and further objects thereof, reference is hadtothe'following description taken inconnection with the accompanying drawings, and its scope will be pointed out inthe appended claims.

Inthe accompanying drawings, Fig. 1 is a circuit diagram, partially schematic, of a televisiontransmittng system including a'cathode-ray tube signal-genemting` system embodying thepresent invention `and Fig. 2 is a group of 'curves-repreindicated. For developing scanning currents forV the signal-generating tube, a line-scanning sawtooth generator I2 and a frame-scanning sawtooth generator I3 are provided, their output circuits being connected to the scanning elements of the cathode-ray tube II, as shown. In order to provide pedestal impulses or pedestal-signal voltages for4 suppressing undesirable signal impulses during theretrace scanning periods, ensuring theproper .form' of the signal to be transmitted-in accordancewith'the present invention, as further described hereinafter, and for blocking out'the cathode, ray `during the retrace periods, there-is provided apedestal impulse generator I4 having its output circuitconnected to the control grid of the signal-'generating tube I I by a suitable 1ead.I5. Ylordeveloping synchronizing impulses for the signal to be transmitted, a synchronizing impulse'generator I'I is provided. The form of combinedlineand frame-synchronizing impulses developed by the generator I1 is controlled by voltage-fromV generator I 4 supplied to generator I1 via a suitable connection I9. For the purpose of'synchronizing the generatorsV I 2, I3, I4, and VI'l,'there-is provided a timing impulse generator 20, preferably stabilized by means of a connection 2I'V to a-suitable source Vof periodic voltage, as, for examplato power mains or to the synchronizing'volt-a-ge source of a motion picture mechanism, where such is employed, The output circuit of thegenerator 2U i-s connected to the several other generators, as shown.

f For the purpose of providing a separate aperiodic, bias voltage having an-amplitude varying solely in-accordance with background-illumination, there `.is provided a conventional generator 22 comprising a photoelectric vtube- 23 having its outputcircuitconnected to the input circuit of adirect current amplier 24, as shown, the output circuit of the amplier including a resistor 25 acrosswhich'the background-illumination control voltage is developed. Y Y Y Connected inV cascade to the output circuit of thesignal-generating tube Vand camera device I0, inthe 'order named, are a video-frequency arnplier 28,'avacuum-tubemixing amplier 25 1ncludingaplurality of control electrodes and an output'circuit, a modulation amplifier 30, a modulator'3I which is alsocoupled to an oscillator 32, as shown, a'power,Y amplifier 33, and an antenna system* 34, 35, all according to conventional practices The output circuit of the generator .'22 is coupled to the i'lrst or control grid of amplier 29, thereby to'utilize the unidirectional control bias developed by generator-22 to adjust the zero axis of the video-signal wave applied to amplifier 29. Synchronizing impulses are also applied to modulationl amplifier -from -generator I'I in accordance ,with conventional practice. Operating voltages forv the tubes VVI I, 23, 24, and 29 are supplied from batteries, as indicated, or any other suitable Sources Neglecting for the moment the details of arrangement and operation of the amplifier 29 and the parts of the system associated therewith in accordance With the present invention, the system just described comprises the elements of a television-transmitting system of conventional design and the various parts thereof illustrated schematically being of any well-known construction, a detailed description of the general system c and its operation is unnecessary herein. Briefly, however, the image of a scene to be transmitted is focused on the target of the tubeA II in Which a cathode ray or beam is developed, focused, and accelerated toward the target. Scanning or deilecting currents developed by the generators I2 and I3 are applied to the scanning coils of the generating tube Il to provide magnetic fields which serve to deflect the cathode ray horizontally and vertically, thereby to scan successive series yof parallel lines, or frames, on the target. The deflecting currents and, hence, the magnetic elds are of Well-known saw-tooth Wave form providing a relatively slowv linear trace and rapid retrace. Thenumber of lines per frame are determined by the relative frame and line frequencies. Pedestal impulses developed by the generator I4 are applied to a control electrode of the tube I I to suppress or block out the beam during the retrace portions of the Ascanning cycles and are applied to the amplifier 29 in a manner and for a purpose presently tobe described. y d

synchronizing impulses developedin the generator il are applied to the amplier 30, while timing impulses developed in the generator 20, stabilized from the power line connection 2|, are applied to the generators I2, I 3, I4, and I'I to lock these generators in synchronism.

The photosensitive elements ofthe target being electrically affected to an extent depending upon the varyingvalues of light at the corresponding incremental areas of the image focused thereon, as the cathode ray passes over the target, a voltage of correspondingly varying amplitude is developed in the-output circuitofrthe signalgenerating tube II and applied to the amplifier 28. The average background illumination of the scene causes the photocell 23 of the generator 22 to develop invits output circuit a'negativebias voltage proportional tothis 'average illumination and this Vvoltage is appliedgto the control electrode of thedirect current amplifier tube 24, thereby to develop across the resistor 25 a unidirectional voltage component which increases positively in proportion tothe background illumination. The video-frequency voltages in the output circuit of the amplier `28'and the background-illumination voltage( developed across the resistor 25 are applied to the amplier 29. These voltages are combinedland amplified in the amplier 29 and translatedV to the amplier Sil, wherein they are further amplified and vmixed With the synchronizing impulses applied from the generator I'I. The modulation voltages are then supplied to the modulator 3|, wherein they are impressed on the carrier wave generated by the oscillator 32 and the resultant modulated-carrier signal is delivered to the power amplifier 33 for amplification and thereupon impressed upon the antenna system 34, 35 to be broadcast.`

Referring now more particularly tothe parts of the system embodying the present invention, the amplier 29, in the preferred embodiment of the invention, comprises a pentode vacuum tube having the usual cathode, anode, control grid, screen, and suppressor. VThe control grid-'cathode voltages.V

circuitcf the tube `29 "is connected to the output 'circuit of.` the amplifier 28 lby wayfof a suitable coupling condenser 36' and "leak resistor; 31,` `while i a -iilterf resistor? Sii-and inductance 40 are included inthe anode Ycircuit-of this tube, "The anode circuit is coupled tothe input circuitof Vthe videoffellllcy amplierll by means of av4 suitable coupling condenser 4 I.

`nectedrto` theresistor 31 vforl applying the unidirectional voltage to the controlgrid of the tube 29. Pedestal impulses are supplied b ya lead `Iii from the generatorV Ill` and a suitable coupling condenser 44 to the suppressorof thetubeA 29`to reduce the conductance Vof tube 29 substantially "to'zero during periods corresponding to=retrace scanning periods, Thus, the videoffrequency `vo`ltage`s, background-illumination voltage, and

pedestal-imrnilse voltage are all applied simultaneously to the amplifier 29. to be combined thereinand thereby, develop in the output `circuit" Qfampliiier 29 a composite signal including the A vided-signal fand lbackgroundillumination "The perationof` the systernQinaccordance with the present invention may best'beexplainedby reference to' the curves of Fig.j2 Iv-"lere,` the sev-` .eral -curves illustrate the waveforms of lsignals developed at idiierent pointsin the system.` Only a few line-frequency cycles are shown, the broken lines indicating tlie'omitted impulses. Thecurves W, G; and B represeritthe videofrequencylvolt vages developed by the .cathode-ray tube Hand amplified 'by and appearing .in the output circuit of the `ampliiierA 28 for a perfectly white background having a vertical black bar; for a grey background having two adjacent vertical bars, one White and one black; for a black background having a vertical white bar, respectively, each line including a sudden change from positive to negative to represent a sudden change from white to black and/or vice versa. At the end of each line period there is a negative impulse, as indicated at N, occurring during the block-out of the cathode ray during the retrace scanning period. There is also shown, during the retrace intervals, a positive voltage surge, as indicated at S. The impulses N and S are, of course, the undesirable impulse surges which must be removed from the signal. The signal at this point of the system does not include the background-illumination unidirectional voltage component.

Curve A illustrates the grid-voltage plate current characteristic of the tube 29 which, it will be noted, is typical of a tube of the linear, sharp cutoff type. Since both the video-frequency and the background-illumination voltages are applied to the control grid of this tube, the combined voltage appearing thereon is of the form illustrated by the curves W1, Gi, and B1. More particularly, the grid of the tube 29 is initially biased so that, with no background bias voltage applied thereto, it operates approximately at cutoff, as indicated by curves B and B1. The unidirectional background bias voltage, being applied positively and increasing in accordance with increases in the average background illumination of the scene, adjusts the point of operation or the position of the alternating current axis of the video-frequency voltages with respect to the curve A accordingly. Hence, for a black background subject, as represented by the video-frequency voltages Bi, with no background bias voltnumber of stages.

'age appliedilthepointof operation orbias for the tube 12eis indicated `at isb. Foivthe grey-and white subjects, thelbackground' biasvoltag'e is increased" accordingly 'and operation i's-gat-'the points Ey and }Ew,`j respectively. -4 Curves 'B representfthe pedestal impulses developed by the genonly necessary that it is suiiicient'completely to cut oil?` the plate current `of the Vtube during the retrace intervals; In a tube in which 'the' supi pressor lgrid has fa substantial control, th'ewave form of the pedestal should, `howeven-be more perfect and yshould be very flat during the trace portionlof the cycle in order that no` shading of thelines willfbe produced thereby. l

With the video-frequency, background-control, and pedestal voltages thus appliedtol the control electrodesgof the amplifier tube 29, the plate current of this tube is of a wave iorm such as representedby thecurves W2, G2, and B2. It will'be appreciatedthfat, when this signal is impressed upon a succeeding stagethrough an alternating current coupling, the Aunidirectional background voltage component is lost. The expression alternating and direct current ampliemstagef as employed herein and in the appended claims, is intended to kdenote any amplifier "arrangement wherein both the unidirectional andalt'ernating voltage components are transmitted, which arrangement may, of` course'include any'number of amplilier tubes coupled for bloth unidirectional o andperiodic voltages. The signal, as'represented 35` by W12, Gg, and B2, therefore, is impressedupon the `ampliiier 35 which mayinclude any "desired The synchronizingsignal impulses, indicated at I, are also applied fromthe generator Il to a suitable stage of the amplifier 3U so that these impulses have the same polarity as the pedestal impulses and video impulses representing black. The final modulation voltage developed by the system and appearing in the output circuit of the amplifier 30, therefore, is of the form represented by the curves W3, Gs, and B3 and this voltage is utilized to modulate the carrier developed by oscillator 32, as mentioned above.

Due to the translation of the signal through the alternating current coupling, such as condenser 4l, the unidirectional voltage component is, of course, lost. However, a backgroundillumination control component is still included in the signal. That is, the variation in the peak values of the combined pedestal and synchronizing impulses with respect to the axis of the modulation wave is representative of the backgroundillumination variations and may be utilized at a receiver to provide a unidirectional voltage corresponding to the voltage developed by the generator 22, to provide background-illumination control.

While the suppressor of the amplier 29 has been found a highly satisfactory point for the application of the pedestal voltages, it will be clear that these voltages may be applied to various other electrodes, such as the control grid or the plate of this ampliiier, by suitably modified circuit arrangements. Moreover, if desired, multigrid tubes of different types may be substituted for the amplifier 29 to provide separate control electrodes for the video-frequency, backgroundillumination, and pedestal voltages or, if desired, all of these voltages may be applied to a single control electrode of a suitable tube, all such modified arrangements being Well within the `understanding of those skilled in the `art following the teachings above set forth.

It will thus be seen that theY present invention provides an efficient and relatively simple system wherein an undistorted signal is developed which is substantially free from undesired impulses dur- .ingthe retrace scanning periods and which includes a background-illumination modulation signal component.

It will be clear that theoperation of the system Vduring the frame retrace interval, when the pedestal voltage is negative during the time required to sean a plurality of lines, is similar to that during lineretrace intervals excepting that the form of the undesired impulses are somewhat different. The Wave forms during these periods have not been illustrated in the interest of brevity.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled Vin the art that various changes and modifications of the tube for developing video-signal voltages, -means for developing pedestal-signal voltages during retrace scanning periods, means for developing a separate aperiodic bias voltage having an amplitude varying solely in accordance with background-illumination, a vacuum-tube ampliand an output circuit, means for applyingl said lvideo-signal voltages vto one of said electrodes,

means for applying said pedestal signals to one of said electrodesy to reduce the conductance of said tubeV substantially to Zero during periods correspondingv to said retrace scanning periods, and means for applying said background-illumination voltage as a bias to said rstone of said control electrodes to adjust the zero axis of the wave applied to said amplier, thereby to develop in said output circuit a composite signalincluding said video-signal and background-illumination voltages. y 2. In a television-transmittingsystem, a cath ode-ray tube signal-generating system comprising means including a scanning system for electing traces and retraces of the ray on the target means for developing pedestal-signal voltages during retracescanning periods, means for developing a separate aperiodic bias voltage varying solely in accordance with background-illumination, a vacuum-tube amplifier having anoutput circuit and comprising a plurality of vcontrol electrodes including a control grid and a suppressor grid, means for applying said video-signal voltages to said control grid, means for applying said pedestal-signal voltages to said suppressor gridto Areduce the conductance of said tube substantially to zero during periods corresponding to said retrace scanning periods, and means for applying said background-illumination voltage as a bias to said controlgrid to adjust the zero axis of the Wave applied to said amplifier, thereby to develop in said output circuit a composite signal including said Vvideo-signal and background-illumination voltages. HAROLD M. LEWIS.

CERTIFICATE OE CORRECTION. Patent No; 2,2LL7,511. July 1, 19m.

HAROLD M. LEwTs.

It is hereby certified that lerror' appears in the `printed. specification of the above numbered patent requiring Correction as follows: Page 1|., ysecond column, line '18, claim- 2, v after "target" insert the words and comma --of the tube for developing video-signal voltages,`; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the. case in the Patent Office.

Signed and sealed this 7th day of October, A. D. 19141.

l Henry Van Arsdale, (Seal) Acting Commseioner of -Patents. 

